Mass spectrometry probes and systems for ionizing a sample transport

ABSTRACT

The invention generally relates to mass spectrometry probes and systems for ionizing a sample. In certain embodiments, the invention provides a mass spectrometry probe including a substrate in which a portion of the substrate is coated with a material, a portion of which protrudes from the substrate.

RELATED APPLICATIONS

The present application claims the benefit of and priority to each ofU.S. provisional patent application Ser. No. 61/926,713, filed Jan. 13,2014, and Indian patent application number 6137/CHE/2013, filed Dec. 28,2013, the content of each of which is incorporated by reference hereinin its entirety.

GOVERNMENT SUPPORT

This invention was made with government support under CHE1307264 awardedby the National Science Foundation. The government has certain rights inthe invention.

FIELD OF THE INVENTION

The invention generally relates to mass spectrometry probes and systemsfor ionizing a sample.

BACKGROUND

Recent progress in mass spectrometry has depended heavily on advances inmethods of ion formation. Creation of stable molecular ions of complexmolecules with minimum internal energy has been a primary goal of suchexperiments. The most widely used methods to achieve this areelectrospray ionization (ESI) and matrix-assisted laser desorptionionization (MALDI). The newer ambient ionization methods, such asdesorption electrospray ionization (DESI), allow samples to be examinedin their native state with minimal or no sample pre-treatment. Theseadvantages and the resulting speed of analysis have led to theintroduction of some fifty different variants of ambient ionization.Direct analysis in real time (DART), extractive electrospray ionization(EESI), desorption atmospheric pressure chemical ionization (DAPCI),desorption atmospheric pressure photoionization (DAPPI), laser ablationelectrospray ionization (LAESI), and paper spray ionization, are some ofthe new methods introduced over the past decade.

Many of those methods use a high voltage source coupled to the probe toachieve ionization in an ambient environment. The application of highvoltage can sometimes cause unwanted fragmentation of a target analyteduring the ionization process.

SUMMARY

The invention provides a low voltage mass spectrometry probe configuredto generated ions without the need for a high voltage source. Aspects ofthe invention are accomplished with a substrate in which a portion ofthe substrate is coated with a material, a portion of which protrudesfrom the substrate. Generally, these protrusions are on the nanoscaleand, without being limited by any particular theory or mechanism ofaction, act as an electrode. The protrusions provide a field strengthhigh enough to cause field emission of microscale solution dropletscontaining analyte at these nanoscale protrusions. In that manner, massspectrometry probes of the invention are able to ionize a target analytethrough the application of a low voltage (e.g., 3 volts or less), ratherthan a high voltage, allowing for ionization without unwantedfragmentation of the target analyte.

In certain aspects, the invention provides a mass spectrometry probeincluding a substrate. The substrate can be porous or nonporous. Anexemplary substrate is a paper substrate, such as a substrate composedof filter paper. The substrate can have any shape. In certainembodiments, the substrate tapers to a tip, such as a substrateincluding a planar portion that tapers to a tip. An exemplary shape is atriangular substrate that tapers to a tip.

The probe further includes a material that coats a portion of thesubstrate. A portion of that material protrudes from the substrate. Incertain embodiments, the material is an electrically conductivematerial, although that is not required, because a solvent surroundingthe material can be an electrolyte in some cases. Any electricallyconductive material may be used with probes of the invention. Anexemplary material includes electrically conductive nanotubes, such ascarbon nanotubes. Typically, the carbon nanotubes coat an externalsurface of the substrate, and a distal portion of a plurality of thecarbon nanotubes protrude from the surface of the substrate. It is alsopossible for portions of the carbon nanotubes, or any chosen material,to impregnate the substrate.

In certain embodiments, the substrate is coupled to a voltage source. Incertain embodiments, the voltage source is configured to generate avoltage of 3 volts or less, such as 2.9 volts or less, 2.8 volts orless, 2.7 volts or less, 2.6 volts or less, 2.5 volts or less, 2.4,volts or less, 2.3 volts or less, 2.2, volts or less, 2.1 volts or less2 volts or less, 1.5 volts or less, or 1 volt or less.

Another aspect of the invention provides a system including a massspectrometry probe having a substrate in which a portion of thesubstrate is coated with a material, a portion of which protrudes fromthe substrate. A voltage source is coupled to the substrate, and a massanalyzer is operably associated with the system such that it receivesions generated from the mass spectrometry probe. In certain embodiments,the probe is discrete from a flow of solvent. In certain embodiments,the probe operates without pneumatic assistance. The mass analyzer canbe for a bench-top mass spectrometer or a miniature mass spectrometer.

Another aspect of the invention provides a system including a massspectrometry probe having carbon nanotubes, a voltage source coupled tothe probe, and a mass analyzer. In certain embodiments, the carbonnanotubes alone make-up the probe. In other embodiments, the probefurther includes a substrate (porous or non-porous) and the carbonnanotubes are coupled to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 panel A is a schematic diagram of ionization from carbon nanotube(CNT) paper. Panel B is a photograph of the ionization source showing apaper triangle and battery along with a grounding electrical connection.Panel C is a mass spectra of triphenylphosphine (M) at 3 kV, 3 V and 1 Vfrom wet CNT paper. Panel D is a field emission-scanning electronmicroscopy (FE-SEM) image of CNT-coated paper. Panel E is an isotopedistribution of the protonated molecule at 3 V. Panel F is a product ionMS² of m/z 263.

FIG. 2 is a full range mass spectrum of triphenylphosphine at 3 V.

FIG. 3 is an ESI mass spectrum (MeOH:H₂O, 1:1) of triphenylphosphine at3 kV. The spectrum shows an enhanced oxidation peak at m/z 279 and itsC₆H₆ fragment at m/z 221, in comparison to the CNT-coated paper (FIG. 1panel C). MS/MS spectrum is shown in the inset.

FIG. 4 is a spectrum produced using rectangular CNT-coated paper and theinset shows the schematic of the paper (with mass spectrometer facingone side of it).

FIG. 5A shows the variation of intensity of the m/z 263 peak withvoltage for CNT-coated paper. FIG. 5B shows the same for normal paper.

FIG. 6 panels A-D show intensity enhancement upon the addition ofdiluted HCl for various analytes (M) at 3 V. Panel A showstriphenylphosphine. Panel B shows tributylphosphine. Panel C showsdiphenylamine. Panel D shows triethylamine. The upper and lower tracesin each plot are on the same scale and show spectra before and after HCladdition, respectively.

FIG. 7 panels A-B show analysis of preformed ions (positive and negativeion modes) at 3 V. Panel A shows tetramethylammonium chloride. Panel Bshows tetramethylammonium bromide.

FIG. 8 panels A-B show Analysis of preformed ions (positive and negativeion modes) at 3 V. Panel A shows tetramethylammonium nitrate. Panel Btetrabutylammonium iodide.

FIG. 9 panels A-C show detection of pesticides examined individuallyfrom the surface of an orange. Panel A is Carbofuran. Panel B is methylparathion. Panel C is parathion.

FIG. 10 shows analysis of a pesticide mixture at 3 V from the surface ofan orange. Isotopic distribution of the peaks is not clearly visible dueto low intensity.

FIG. 11 panels A-C show analysis of tablets from CNT-coated paper at 3 Vwith their mass spectral and MS² data. Panel A is Crocine (paracetamol).Panel A is xyzal (levocetirizine dihydrochloride). Panel A is combiflam(paracetamol).

FIG. 12 panels A-H show detection of various amino acids (90 ng) loadedon CNT-coated paper and spectra recorded at 3 V. Panel A isphenylalanine. Panel b is methionine. Panel C is glutamic acid. Panel Dis glutamine. Panel E is isoleucine. Panel F is valine. Panel G isproline. Panel H is serine.

FIG. 13 panels A-C show Raman measurement of CNT-coated paper before andafter ionization. Panel A shows neutral molecules (30 ppm TTP inMeOH/H₂O) and preformed ions (tetramethylammonium bromide). Panel Bshows in positive ion mode. Panel C shows in negative ion mode.

FIG. 14 is a schematic illustrating a system of the invention foranalysis of various different types of samples. The mass spectrometryprobe is illustrated to be coated with carbon nanotubes.

FIG. 15 panels A-E show detection of proton bound dimer of various acidsat 1V. Panel A is formic acid. Panel B is acetic acid. Panel C ispropionic acid. Panel D is butyric acid. Panel E is pentanoic acid. Theletter “D” represents proton bound dimer.

FIG. 16 panels A-D show detection of mixed dimer of various acidmixtures. Panel A is acetic acid & butyric acid. Panel B is propionicacid & butyric acid. Panel C is acetic acid & pentanoic acid. Panel D isformic acid & pentanoic acid. The letter “D” represents proton bounddimer.

FIG. 17 panels A-B show D/M ratio vs Voltage study for various acids intwo different solvents. Panel A is in water. Panel B is in methanol.

FIG. 18 shows D/M ratio vs voltage for propionic acid in differentsolvents.

FIG. 19 panels A-D show detection hydrates of various anions at 1V.Panel A is chloride, Panel B is bromide. Panel C is iodide. Panel D isacetate.

FIG. 20 is a plot of signal to noise ratio (S/N) with respect to sprayvoltage for the low voltage spray mass spectrometry (CNT coated paperspray) using 50 ppm triphenylphosphine solution as test analyte. Thisspray was conducted by delivery of 5 μL of analyte solution onto the tipfor each experiment. The CNT coated paper tip was positionedapproximately 1 mm away from the MS inlet.

FIG. 21 is a Plot of signal intensity with respect to spray voltage forthe low voltage spray mass spectrometry (CNT coated paper spray) using50 ppm triphenylphosphine solution in methanol as test analyte;rectangular CNT paper spray, capillary and tube lens voltages 0 V,distance from MS inlet ˜0.5 mm, average of 5 experiments.

FIG. 22 shows negative mode mass spectra obtained from Bacillus subtilisusing CNT paper spray mass spectrometry at 3 V (top) and paper spraymass spectrometry at 3.5 kV (below). Spray solvent (methanol, 10 μL) wasadded to few colonies smeared to the paper rectangle.

DETAILED DESCRIPTION

The invention generally relates to mass spectrometry probes and systemsfor ionizing a sample. In certain embodiments, the invention provides amass spectrometry probe including a substrate in which a portion of thesubstrate is coated with a material, a portion of which protrudes fromthe substrate. With probes of the invention, pneumatic assistance is notrequired to transport an analyte; rather, a voltage (e.g., a low voltageof, for example, 3 volts or less) is simply applied to the substratethat is held in front of a mass spectrometer.

In certain embodiments, the substrate is kept discrete (i.e., separateor disconnected) from a flow of solvent, such as a continuous flow ofsolvent. Instead, sample is either spotted onto the substrate or swabbedonto it from a surface including the sample. The spotted or swabbedsample is then connected to a voltage source to produce ions of thesample which are subsequently mass analyzed. The sample is transportedthrough the substrate without the need of a separate solvent flow.Pneumatic assistance is not required to transport the analyte; rather, avoltage is simply applied to the porous material that is held in frontof a mass spectrometer.

In other embodiments, the substrate is coupled to a continuous solventflow or a solvent reservoir so that the substrate can be continuouslysupplied with solvent. Such an exemplary set-up is described for examplein Bare et al. (International patent application publication number WO2012/170301), the content of which is incorporated by reference hereinin its entirety.

The solvent may assist in separation/extraction and ionization. Anysolvents may be used that are compatible with mass spectrometryanalysis. In particular embodiments, favorable solvents will be thosethat are also used for electrospray ionization. Exemplary solventsinclude combinations of water, methanol, acetonitrile, andtetrahydrofuran (THF). The organic content (proportion of methanol,acetonitrile, etc. to water), the pH, and volatile salt (e.g. ammoniumacetate) may be varied depending on the sample to be analyzed. Forexample, basic molecules like the drug imatinib are extracted andionized more efficiently at a lower pH. Molecules without an ionizablegroup but with a number of carbonyl groups, like sirolimus, ionizebetter with an ammonium salt in the solvent due to adduct formation.

The material of the substrate can be electrically conductive or it canbe an insulator. The substrate can be composed of a porous material or anon-porous material, or a combination thereof. Non-porous refers tomaterials that do not include through-holes that allow liquid or gas topass through the material, exiting the other opposite side. Exemplary,non-porous materials include but are not limited to a metal, a plastic,a polymer, a glass, or a graphene.

Porous materials are described for example in Ouyang et al. (U.S. Pat.No. 8,859,956), the content of which is incorporated by reference hereinin its entirety. In certain embodiments, the porous material is anycellulose-based material. In other embodiments, the porous material is anon-metallic porous material, such as cotton, linen wool, synthetictextiles, or plant tissue (e.g., a leaf). In still other embodiments,the porous material is paper. Advantages of paper include: cost (paperis inexpensive); it is fully commercialized and its physical andchemical properties can be adjusted; it can filter particulates (cellsand dusts) from liquid samples; it is easily shaped (e.g., easy to cut,tear, or fold); liquids flow in it under capillary action (e.g., withoutexternal pumping and/or a power supply); and it is disposable.

In particular embodiments, the porous material is filter paper.Exemplary filter papers include cellulose filter paper, ashless filterpaper, nitrocellulose paper, glass microfiber filter paper, andpolyethylene paper. Filter paper having any pore size may be used.Exemplary pore sizes include Grade 1 (11 μm), Grade 2 (8 μm), Grade 595(4-7 μm), and Grade 6 (3 μm), Pore size will not only influence thetransport of liquid inside the spray materials, but could also affectthe formation of the Taylor cone at the tip. The optimum pore size willgenerate a stable Taylor cone and reduce liquid evaporation. The poresize of the filter paper is also an important parameter in filtration,i.e., the paper acts as an online pretreatment device. Commerciallyavailable ultra-filtration membranes of regenerated cellulose, with poresizes in the low nm range, are designed to retain particles as small as1000 Da. Ultra filtration membranes can be commercially obtained withmolecular weight cutoffs ranging from 1000 Da to 100,000 Da.

Substrates of the invention can be any shape and a sharp point is notrequired to generate ions using probes of the invention. For example,substrates of the invention can be rectangular, and ions can begenerates along an edge of the rectangle, as opposed to a point at acorner of the rectangle. In particular embodiments, the porous materialis shaped to have a macroscopically sharp point, such as a point of atriangle, for ion generation. Probes of the invention may have differenttip widths. In certain embodiments, the probe tip width is at leastabout 5 μm or wider, at least about 10 μm or wider, at least about 50 μmor wider, at least about 150 μm or wider, at least about 250 μm orwider, at least about 350 μm or wider, at least about 400μ or wider, atleast about 450 μm or wider, etc. In particular embodiments, the tipwidth is at least 350 μm or wider. In other embodiments, the probe tipwidth is about 400 μm. In other embodiments, probes of the inventionhave a three dimensional shape, such as a conical shape. In certainembodiments, the substrate tapers to a tip, such as a substrateincluding a planar portion that tapers to a tip. An exemplary shape is atriangular substrate that tapers to a tip.

With probes of the invention, a portion or an entirety of the substrateis coated with a material. The coating is applied to the substrate suchthat one or more portions of the material protrude from the substrate.Generally, these protrusions are on the nanoscale (nanometer features)and, without being limited by any particular theory or mechanism ofaction, act as an electrode. The protrusions provide a field strengthhigh enough to cause field emission of microscale solution dropletscontaining analyte at these nanoscale protrusions. In that manner, massspectrometry probes of the invention are able to ionize a target analytethrough the application of a low voltage (e.g., 3 volts or less), ratherthan a high voltage, allowing for ionization without unwantedfragmentation of the target analyte.

In certain embodiments, the material is an electrically conductivematerial, although that is not required, because a solvent surroundingthe material can be an electrolyte in some cases. Any electricallyconductive material may be used with probes of the invention. Anexemplary material includes electrically conductive nanotubes. Nanotubesare nanometer-scale tube-like structure. Exemplary nanotubes are carbonnanotubes, silicon nanotubes, boron nitride nanotubes, or inorganicnanotubes (i.e., nanotubes formed of metal oxides, or groupIII-Nitrides). Typically, the nanotubes coat an external surface of thesubstrate, and a portion (such as a distal portion) of a plurality ofthe nanotubes protrude from the surface of the substrate. It is alsopossible for portions of the nanotubes, or any chosen material, toimpregnate the substrate.

In certain embodiments, the nanotubes are carbon nanotubes, which aredescribed for example in Monthioux et al. (Carbon 44 (9): 1621, 2006),Oberlin et al. (Journal of Crystal Growth 32 (3): 335-349, 1976), Endoet al. (Carbon 37 (11): 1873, 2002), Izvestiya wet al. (Metals. 1982,#3, pp. 12-17), Tennent (U.S. Pat. No. 4,663,230), Iijima et al. (Nature354 (6348): 56-58, 1991), Mintmire et al., (Phys. Rev. Lett. 68 (5):631-634, 1992), Bethune (Nature 363 (6430): 605-607, 1993), Iijima etal., (Nature 363 (6430): 603-605, 1993), Krätschmer et al. (Nature 347(6291): 354-358, 1990), and Kroto et al. (Nature 318 (6042): 162-163,1985), thwe content of each of which is incorporated by reference hereinin its entirety.

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindricalnanostructure. Carbon nanotubes are members of the fullerene structuralfamily. Their name is derived from their long, hollow structure with thewalls formed by one-atom-thick sheets of carbon, called graphene. Thesesheets are rolled at specific and discrete (“chiral”) angles, and thecombination of the rolling angle and radius decides the nanotubeproperties; for example, whether the individual nanotube shell is ametal or semiconductor. Carbon nanotubes are categorized assingle-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), andboth may be used with probes of the invention.

Most single-walled nanotubes (SWNT) have a diameter of close to 1nanometer, with a tube length that can be many millions of times longer.The structure of a SWNT can be conceptualized by wrapping aone-atom-thick layer of graphite called graphene into a seamlesscylinder. The way the graphene sheet is wrapped is represented by a pairof indices (n,m). The integers n and m denote the number of unit vectorsalong two directions in the honeycomb crystal lattice of graphene. Ifm=0, the nanotubes are called zigzag nanotubes, and if n=m, thenanotubes are called armchair nanotubes. Otherwise, they are calledchiral. The diameter of an ideal nanotube can be calculated from its(n,m) indices as follows:

${d = {{\frac{a}{\pi}\sqrt{\left( {n^{2} + {nm} + m^{2}} \right)}} = {78.3\sqrt{\left( {\left( {n + m} \right)^{2} - {nm}} \right)}{pm}}}},$

where a=0.246 nm.

Multi-walled nanotubes (MWNT) consist of multiple rolled layers(concentric tubes) of graphene. There are two models that can be used todescribe the structures of multi-walled nanotubes. In the Russian Dollmodel, sheets of graphite are arranged in concentric cylinders, e.g., a(0,8) single-walled nanotube (SWNT) within a larger (0,17) single-wallednanotube. In the Parchment model, a single sheet of graphite is rolledin around itself, resembling a scroll of parchment or a rollednewspaper. The interlayer distance in multi-walled nanotubes is close tothe distance between graphene layers in graphite, approximately 3.4 Å.The Russian Doll structure is observed more commonly. Its individualshells can be described as SWNTs, which can be metallic orsemiconducting. Because of statistical probability and restrictions onthe relative diameters of the individual tubes, one of the shells, andthus the whole MWNT, is usually a zero-gap metal.

As used herein, the term carbon nanotubes includes carbon nanobuds,which are a combination of carbon nanotubes and fullerenes. In carbonnanobuds, fullerene-like buds are covalently bonded to the outersidewalls of the underlying carbon nanotube. This hybrid material hasuseful properties of both fullerenes and carbon nanotubes. Inparticular, they have been found to be exceptionally good fieldemitters. In composite materials, the attached fullerene molecules mayfunction as molecular anchors preventing slipping of the nanotubes, thusimproving the composite's mechanical properties.

As used herein, the term carbon nanotubes also includes graphenatedCNTs, which are a hybrid that combines graphitic foliates grown alongthe sidewalls of multiwalled or bamboo style CNTs. Graphenated CNTs aredescribed for example in Yu et al. (J. Phys. Chem. Lett. 13 2 (13):1556-1562, 2011), and Stoner et al. (Appl. Phys. Lett. 18 99 (18):183104, 2011), the content of each of which is incorporated by referenceherein in its entirety.

As used herein, the term carbon nanotubes also includes: doped carbonnanotubes, such as nitrogen doped carbon nanotubes (Kouvetakis et al.,Chemistry of Materials 6 (6): 811, 1994; Zhong et al., Journal ofPhysics and Chemistry of Solids 71 (2): 134, 2010; Yin et al., AdvancedMaterials 15 (21): 1840, 2003; and Oku et al., Diamond and RelatedMaterials 9 (3-6): 906, 2000); and a carbon peapod, which is a hybridcarbon material which traps fullerene inside a carbon nanotube (Smith etal., Nature 396: 323-324, 1998; and Smith et al., Chem. Phys. Lett. 321:169-174, 2000).

An exemplary system using a carbon nanotube probe of the invention isshown in FIG. 14. As described in the Examples below, a carbon nanotube(CNT) coated/impregnated paper can be used to generate ions from organicmolecules at potentials at 3 V or lower. Common pesticides from thesurface of an orange, ingredients of medicinal tablets and a variety ofanalytes such as amino acids have been characterized using probes andsystems of the invention.

In certain embodiments, the material is an electrically conductivefibrous material, such that portions of a plurality of the fibersprotrude from the substrate. Exemplary fibers are metal or carbonfibers, or metal or carbon nanowires. Those fibers can be hollow orsolid. In certain embodiments, the protrusions are printed as nanometerfeatures (electrically conductive or not based on the material used inthe process) onto the substrate (a printed coating). Dip pennanolithography can be used for this process, as described in Ginger etal. (Angewandte Chemie International Edition 43 (1): 30-45, 2004) orother known processes may be used, such as described for example inYueh-Lin et al. (Appl. Phys. Lett., 81(3):562-564, 2002), the content ofeach which is incorporated by reference herein in its entirety.

Mass spectrometry probes of the invention are typically coupled to avoltage source. In certain embodiments, the voltage source is a lowvoltage source, that is, a voltage source configured to generate avoltage of 3 volts or less, such as 2.9 volts or less, 2.8 volts orless, 2.7 volts or less, 2.6 volts or less, 2.5 volts or less, 2.4,volts or less, 2.3 volts or less, 2.2, volts or less, 2.1 volts or less2 volts or less, 1.5 volts or less, or 1 volt or less. In otherembodiments, a high voltage source (e.g., greater than 3 volts) iscoupled to the probes of the invention. Such couplings are known in theart.

Mass spectrometry probes of the invention can be interfaced with massspectrometers for analysis of samples. As mentioned above, no pneumaticassistance is required to transport the droplets. Ambient ionization ofanalytes is realized on the basis of these charged droplets, offering asimple and convenient approach for mass analysis of solution-phasesamples. Sample solution is directly applied on the probe held in frontof an inlet of a mass spectrometer without any pretreatment. Then theambient ionization is performed by applying a potential (high or low) tothe probe.

Any type of mass spectrometer known in the art can be used with provesof the invention. For example, the mass spectrometer can be a standard,bench-top mass spectrometer. In other embodiments, the mass spectrometeris a miniature mass spectrometer. An exemplary miniature massspectrometer is described, for example in Gao et al. (Z. Anal. Chem.2006, 78, 5994-6002), the content of which is incorporated by referenceherein in its entirety In comparison with the pumping system used forlab-scale instruments with thousands watts of power, miniature massspectrometers generally have smaller pumping systems, such as a 18 Wpumping system with only a 5 L/min (0.3 m3/hr) diaphragm pump and a 11L/s turbo pump for the system described in Gao et al. Other exemplaryminiature mass spectrometers are described for example in Gao et al.(Anal. Chem., 80:7198-7205, 2008), Hou et al. (Anal. Chem.,83:1857-1861, 2011), and Sokol et al. (Int. J. Mass Spectrom., 2011,306, 187-195), the content of each of which is incorporated herein byreference in its entirety. Miniature mass spectrometers are alsodescribed, for example in Xu et al. (JALA, 2010, 15, 433-439); Ouyang etal. (Anal. Chem., 2009, 81, 2421-2425); Ouyang et al. (Ann. Rev. Anal.Chem., 2009, 2, 187-214); Sanders et al. (Euro. J. Mass Spectrom., 2009,16, 11-20); Gao et al. (Anal. Chem., 2006, 78(17), 5994-6002); Mulliganet al. (Chem.Com., 2006, 1709-1711); and Fico et al. (Anal. Chem., 2007,79, 8076-8082).), the content of each of which is incorporated herein byreference in its entirety.

In certain embodiments, systems of the invention are equipped with adiscontinuous interface, which is particularly useful with miniaturemass spectrometers. An exemplary discontinuous interface is describedfor example in Ouyang et al. (U.S. Pat. No. 8,304,718), the content ofwhich is incorporated by reference herein in its entirety. In certainembodiments, it is advantage to heat the sample during analysis.Accordingly, in certain embodiments, mass spectrometry probes of theinvention are configured with a heating element, such as described inCooks et al. (U.S. patent application publication number 2013/0344610),the content of which is incorporated by reference herein in itsentirety.

In certain embodiments, methods and systems of the invention use aporous material, e.g., paper, to hold and transport analytes for massspectral analysis. Analytes in samples are pre-concentrated, enrichedand purified in the porous material in an integrated fashion forgeneration of ions with application of a voltage (low or high) to theporous material. In certain embodiments, a discrete amount of transportsolution (e.g., a droplet or a few droplets) is applied to assistmovement of the analytes through the porous material. In certainembodiments, the analyte is already in a solution that is applied to theporous material. In such embodiments, no additional solvent need beadded to the porous material. In other embodiments, the analyte is in apowdered sample that can be easily collected by swabbing a surface.Systems and methods of the invention allow for analysis of plant oranimal tissues, or tissues in living organisms.

Methods and systems of the invention can be used for analysis of a widevariety of small molecules, including epinephrine, serine, atrazine,methadone, roxithromycin, cocaine and angiotensin I or molecularcomplexes (e.g., protein and peptide complexes). All display highquality mass and MS/MS product ion spectra from a variety of poroussurfaces. Methods and systems of the invention allow for use of smallvolumes of solution, typically a few μl, with analyte concentrations onthe order of 0.1 to 10 μg/mL (total amount analyte 50 pg to 5 ng) andgive signals that last from one to several minutes.

Methods and systems of the invention can be used also for analysis of awide variety of biomolecules, including proteins and peptides andbimolecular complex (protein or peptide complexes). Methods of theinvention can also be used to analyze oligonucleotides from gels. Afterelectrophoretic separation of oligonucleotides in the gel, the band orbands of interest are blotted with porous material using methods knownin the art. The blotting results in transfer of at least some of theoligonucleotides in the band in the gel to the probes of the invention.The probe is then connected to a voltage source and the oligonucleotidesare ionized and sprayed into a mass spectrometer for mass spectralanalysis.

Methods and systems of the invention can be used for analysis of complexmixtures, such as whole blood or urine. The typical procedure for theanalysis of pharmaceuticals or other compounds in blood is a multistepprocess designed to remove as many interferences as possible prior toanalysis. First, the blood cells are separated from the liquid portionof blood via centrifugation at approximately 1000×g for 15 minutes(Mustard, J. R; Kinlough-Rathbone, R. L.; Packham, M. A. Methods inEnzymology; Academic Press, 1989). Next, the internal standard is spikedinto the resulting plasma and a liquid-liquid or solid-phase extractionis performed with the purpose of removing as many matrix chemicals aspossible while recovering nearly all of the analyte (Buhrman, D. L.;Price, P. I.; Rudewicz, P. J. Journal of the American Society for MassSpectrometry 1996, 7, 1099-1105). The extracted phase is typically driedby evaporating the solvent and then resuspended in the a solvent used asthe high performance liquid chromatography (HPLC) mobile phase(Matuszewski, B. K.; Constanzer, M. L.; Chavez-Eng, C. M., Ithaca, N.Y.,Jul. 23-25 1997; 882-889). Finally, the sample is separated in thecourse of an HPLC run for approximately 5-10 minutes, and the eluent isanalyzed by electrospray ionization-tandem mass spectrometry(Hopfgartner, G.; Bourgogne, E. Mass Spectrometry Reviews 2003, 22,195-214).

Methods and systems of the invention avoid the above sample work-upsteps. Methods and systems of the invention analyze a dried blood spotsin a similar fashion, with a slight modification to the extractionprocedure. First, a specialized device is used to punch out identicallysized discs from each dried blood spot. The material on these discs isthen extracted in an organic solvent containing the internal standard(Chace, D. H.; Kalas, T. A.; Naylor, E. W. Clinical Chemistry 2003, 49,1797-1817). The extracted sample is dried on the paper substrate, andthe analysis proceeds as described herein.

Methods and systems of the invention can directly detect individualcomponents of complex mixtures, such as caffeine in urine, 50 pg ofcocaine on a human finger, 100 pg of heroin on a desktop surface, andhormones and phospholipids in intact adrenal tissue, without the needfor sample preparation prior to analysis. Methods and systems of theinvention allow for simple imaging experiments to be performed byexamining, in rapid succession, needle biopsy tissue sectionstransferred directly to paper.

Analytes from a solution are applied to the probe for examination andthe solvent component of the solution can serve as the electrospraysolvent. In certain embodiments, analytes (e.g., solid or solution) arepre-spotted onto the porous material, e.g., paper, and a solvent isapplied to the material to dissolve and transport the analyte into aspray for mass spectral analysis.

In certain embodiments, a solvent is applied to the porous material toassist in separation/extraction and ionization. Any solvents may be usedthat are compatible with mass spectrometry analysis. In particularembodiments, favorable solvents will be those that are also used forelectrospray ionization. Exemplary solvents include combinations ofwater, methanol, acetonitrile, and THE The organic content (proportionof methanol, acetonitrile, etc. to water), the pH, and volatile salt(e.g. ammonium acetate) may be varied depending on the sample to beanalyzed. For example, basic molecules like the drug imatinib areextracted and ionized more efficiently at a lower pH. Molecules withoutan ionizable group but with a number of carbonyl groups, like sirolimus,ionize better with an ammonium salt in the solvent due to adductformation.

In certain embodiments, a multi-dimensional approach is undertaken. Forexample, the sample is separated along one dimension, followed byionization in another dimension. In these embodiments, separation andionization can be individually optimized, and different solvents can beused for each phase.

In other embodiments, transporting the analytes on the probe isaccomplished by a solvent in combination with an electric field. When anelectric potential is applied, the direction of the movement of theanalytes on the probe is found to be related to the polarity of theircharged forms in solution. Pre-concentration of the analyte before thespray can also be achieved on the probe by placing an electrode at apoint on the probe. By placing a ground electrode near the tip, a strongelectric field is produced through the probe when a DC voltage isapplied, and charged analytes are driven forward under this electricfield. Particular analytes may also be concentrated at certain parts ofthe probe before the spray is initiated.

In certain embodiments, chemicals are applied to the probe to modify thechemical properties of the probe. For example, chemicals can be appliedthat allow differential retention of sample components with differentchemical properties. Additionally, chemicals can be applied thatminimize salt and matrix effects. In other embodiments, acidic or basiccompounds are added to the porous material to adjust the pH of thesample upon spotting. Adjusting the pH may be particularly useful forimproved analysis of biological fluids, such as blood. Additionally,chemicals can be applied that allow for on-line chemical derivatizationof selected analytes, for example to convert a non-polar compound to asalt for efficient electrospray ionization.

In certain embodiments, the chemical applied to modify the porousmaterial is an internal standard. The internal standard can beincorporated into the material and released at known rates duringsolvent flow in order to provide an internal standard for quantitativeanalysis. In other embodiments, the porous material is modified with achemical that allows for pre-separation and pre-concentration ofanalytes of interest prior to mass spectrum analysis.

The spray droplets can be visualized under strong illumination in thepositive ion mode and are comparable in size to the droplets emittedfrom a nano-electro spray ion sources (nESI). In the negative ion mode,electrons are emitted and can be captured using vapor phase electroncapture agents like benzoquinone.

The methodology described here has desirable features for clinicalapplications, including neotal screening, therapeutic drug monitoringand tissue biopsy analysis. The procedures are simple and rapid. Theporous material serves a secondary role as a filter, e.g., retainingblood cells during analysis of whole blood. Significantly, samples canbe stored on the porous material and then analyzed directly from thestored porous material at a later date without the need transfer fromthe porous material before analysis. Systems of the invention allow forlaboratory experiments to be performed in an open laboratoryenvironment.

INCORPORATION BY REFERENCE

References and citations to other documents, such as patents, patentapplications, patent publications, journals, books, papers, webcontents, have been made throughout this disclosure. All such documentsare hereby incorporated herein by reference in their entirety for allpurposes.

EQUIVALENTS

Various modifications of the invention and many further embodimentsthereof, in addition to those shown and described herein, will becomeapparent to those skilled in the art from the full contents of thisdocument, including references to the scientific and patent literaturecited herein. The subject matter herein contains important information,exemplification and guidance that can be adapted to the practice of thisinvention in its various embodiments and equivalents thereof.

EXAMPLES

The Examples below illustrate certain embodiments of the invention inwhich ambient ionization is achieved by spraying from a carbon nanotube(CNT) paper surface under the influence of small voltages (≧1V). Organicmolecules give simple high quality mass spectra without fragmentation inthe positive or negative ion modes. Conventional field ionization isruled out and field emission of microdroplets is indicated. Microscopicexamination of the CNT paper confirms the nanoscale features at themodified paper surface are responsible for high electric fields. Ramanspectra imply substantial current flows in the nanotubes. Analyticalperformance is shown with volatile and non-volatile compounds and avariety of matrices.

Example 1 Materials and Probe Preparation

Multi-walled carbon nanotubes (MWNTs), referred to as CNTs were using toform probes of the invention. They were dispersed in water (2 mg in 25mL water) using sodium dodecyl sulfate (6 mg) as a surfactant (K.Moshammer, F. Hennrich, M. M. Kappes, Nano Res. 2009, 2, 599-606). ThisCNT suspension was drop cast onto Whatman 42 filter paper (3 μL of theCNT suspension coating 5 mm²). Then the paper was dried in air and cutinto triangles with dimensions 2×5 mm (base×height). The CNT-coatedpaper triangle was connected to a 3 V battery and held close (2 mm) tothe mass spectrometer inlet. Then it was loaded with sample (typicallyas 30 ppm solutions). The volume of solvent used was 2 μL and repeatedmeasurements using the same paper used the same aliquot of pure solvent.All measurements were made at 3 V. Mass spectra were recorded in thepositive ion mode for all analytes except preformed ions. For preformedions derived from salts, both positive and negative ion mode spectrawere recorded at ±3V. All spectra were recorded under the followingexperimental conditions: solvent methanol/water (1:1), source voltage±3V, capillary temperature 150° C., capillary voltage ±15 V, and tubelens voltage ±55 V.

Single walled carbon nanotubes (SWNTs) were also used for themeasurements. The following parameters were used for all the ESIexperiments: Source voltage −5 kV, sheath gas (nitrogen) flow rate 8(manufacturer's unit), solvent flow rate 2 μL/min and all otherparameters were same as paper spray. All ESI mass spectra correspond toan average of 100 scans.

SWCNTs and MWCNTs were purchased from Nanocyl s.a, USA; SDS from RFCLLtd., Gujarat, India; triphenylphosphine from Spectrochem Pvt. Ltd.,Mumbai, India; tributylphosphine from Wako Pure Chemical IndustriesLtd.; diphenylamine and triethylamine from Merck Ltd., Mumbai, India.The pesticides, carbofuran, methyl parathion and parathion, werepurchased from Sigma Aldrich, India. All the medical tablets used(Crocin, Combiflam and Xyzal, all trade names) were purchased from alocal pharmacy. Amino acids used in the experiments were purchased fromSisco Research Laboratories Pvt. Ltd., Mumbai, India. All analytes(other than pesticides and tablets) were used at 30 ppm concentration.HPLC grade methanol (Sigma Aldrich) and MeOH/water 1:1 were used assolvents.

All mass spectra were recorded using an ion trap LTQ XL (ThermoScientific, San Jose, Calif.). MS² analysis using collision-induceddissociation was performed to confirm the identity of the ions. Ramanmeasurements were made using a Witec GmbH Confocal RamanMicrospectrometer, Germany with 532 nm and 633 nm laser excitations. AFEI make field emission SEM was used to image the CNT-coated papersamples.

Example 2 Sample Analysis Using Probes of the Invention

This example shows that ionization can be achieved from a substratecoated/impregnated with carbon nanotubes (CNTs) at a potential of just afew volts. The high electric fields produced at the small CNTprotrusions are suggested to be the reason for low voltage ionizationwhich appears to occur by field emission of charged microdroplets (Xu etal., Anal. Chem. 1996, 68, 4244-4253; and Wang et al., Anal. Chim. Acta2000, 406, 53-65). Various analytes applied to the tip of the coatedpaper are detectable in small amounts. Neutral molecules typicallyappear as either their protonated or deprotonated forms while saltsyield both positive and negative ions. The fact that a high voltage (HV)is not needed sets this method apart from other spray ambient ionizationmethods.

Experiments were done with triangles of CNT-coated paper, wet withMeOH/water and connected to a 3 V battery (FIG. 1 panels A-B). Massspectra recorded for triphenylphosphine (TPP) using the CNT paper and 3V battery source (FIG. 1 panel C) exhibited a peak at m/z 263 due toprotonated triphenylphosphine, [M+H]⁺. Spectra could be collected for2-3 sec using 2 μL of solvent. The full range mass spectrum of TPP onCNT-coated paper (FIG. 2) is similar to the ESI mass spectrum recordedat 3 kV (FIG. 3). Although the intensity of the molecular ion at 3 V isas much at 104 times less than that seen in ESI, the conditions are lessharsh, in particular the oxidation product at m/z 279 as well as that ofa trace homologous impurity (product at m/z 293) are absent, as aretheir fragmentation products at m/z 203 and m/z 219. Moreover, the massspectrum shows a well-defined isotopic pattern (FIG. 1 panel E) of themolecular ion and confirmation of its structure comes from tandem massspectrometric data (FIG. 1 panel F) which show the expected benzene lossand associated further loss of H₂ upon collision induced dissociation.

An increase in the applied potential increases the ion intensity,saturating at 4 kV, at which point the signal was almost of the samemagnitude as the ESI signal. However, no additional features wereobserved. The two spectra (at 3 kV and 3 V, both from CNT coated paper)shown in FIG. 1 panel C are identical in terms of the ions observed. Aminimum applied voltage of 3 V is essential for detectable ion signals.Control experiments confirmed the fact that CNTs assisted in theionization process at 3 V. Filter paper (without the CNT-coating) butcut similarly and using the same solvent was not as effective atproducing detectable ions with a range of analytes, even up to 500 V.Closer examination of the edge of the paper shows protruding nanotubes(FIG. 1 panel D). From these results and experiments described below, itis suggested that field emission of microscale solution dropletscontaining analyte occurs at these nanoscale protrusions and this isresponsible for the observed ionization event.

Additional experiments were conducted to explore the mechanism ofionization. Clearly the absence of fragment ions in the mass spectrummay be attributed to the occurrence of soft ionization events. Theoccurrence of ionization at 3 V strongly implies a process associatedwith a very high electric field. The field must be due to the smallconductive CNT structures (FIG. 1 panel D) protruding from the surfaceof the filter paper and acting as an electrode (Gruener wet al., J.Fortagh, Phys. Rev. A: At., Mol., Opt. Phys. 2009, 80). The voltage(from the battery) applied at the CNT electrode induces an electricfield between the paper tip and the mass spectrometer inlet. The fieldintensity is high at the paper tip, where ionization occurs. Todifferentiate the contributions of the protruding CNT structures andmacroscopic paper point to ionization, another experiment was done wherea rectangular piece of CNT-coated paper was held in front of the massspectrometer inlet (with one of the long sides facing the MS inlet) andionization of TPP was attempted. All other parameters except the shapeof the paper were held constant. The mass spectra showed (FIG. 4)ionization of TPP at 3 V from this paper rectangle. This proves thatthere is no involvement in this case of the pointed paper tip but theprotruding CNTs are responsible for ionization, revealing the role ofnano scale CNTs in providing a field strength high enough to cause fieldemission. FIGS. 5A and 5B show the change in intensity of the molecularion peaks with voltage for CNT-coated and normal paper triangles,respectively. The ion signals for both these papers saturate at highvoltages, but with CNTs, the onset of ion ejection is much earlier. Thusfrom these experiments it is reasonable to conclude that at lowervoltages CNTs play a role in ionization and with the increase of voltagea Taylor cone forms at the paper tip is formed and the macroscopicelectric field is responsible for ionization.

In conventional field ionization (Luo et al., Chem. Phys. Lett. 2011,505, 126-129; and Goodsell et al., Arch., Phys. 2010, 1-12), vapor phasemolecules placed in a strong electric field lose an electron to formpositively charged radical cations. Many of the analytes used are simplevolatile organic molecules which if ionized by this mechanism would beexpected to give M⁺ radical cations, e.g. m/z 262 in the case oftriphenylphospine, not the observed m/z 263. To test whether fieldionization of vapour phase triethylamine might contribute triethylamine(vapor pressure, p=57 torr at 20° C.) was dissolved in acetone (p=184.5torr at 20° C.) and introduced into the field (the gap between CNTs andMS inlet) as the vapor and ionization was attempted at low voltages. Theresults showed that analyte vapors do not give detectable ionization. Itwas concluded that field emission occurs from a solvated analyte ordroplet, as in all cases only [M+H]⁺ and not the radical cation M⁺. wasdetected.

To further test the proposed field emission of charged dropletmechanism, the experiment with TPP and three other analytes was repeatedin the presence and absence of added protic acid (Cech, C. G. Enke, MassSpectrom Rev 2001, 20, 362-387). Addition of acid will generate the saltand should inhibit simple field ionization (to give M⁺) but it shouldincrease the field emission/ionization from droplets (to give M+H)⁺. Forthis reason, particular analytes were selected which contain basicfunctional groups (phosphines and amines) and they were analyzed beforeand after addition of dilute acid (HCl). FIG. 6 shows enhancement inrelative intensity of the protonated molecules after the addition ofdilute acid to analytes which contain basic functional groups. Thisenhancement supports ionization of a solvated species.

Various preformed ions (derived from salts tetramethylammonium chloride,tetramethylammonium bromide, tertramethylammonium nitrate andtetrabutylammonium iodide) were studied under the same conditions. Inaccordance with the proposed mechanism, both positive and negative ionswere observed in the CNT-derived mass spectra (FIG. 7 panels A-B andFIG. 8 panels A-B). No fragmentation was observed and the extremesoftness of the process compared even to other soft ionization methodsis indicated by the presence of hydrated halide anions. These studiesshow that preformed ions can also be ejected from the surface indroplets and conventional field ionization is not responsible for ionformation.

To further characterize the CNT-ionization technique, it was employedfor qualitative analysis of various analytes including pesticides,antibiotics and amino acids. All gave characteristic mass spectra; hencethis low voltage ionization methodology is shown to be useful fordiverse analytical needs. Direct analysis of various contaminants onfruits is possible with this methodology. Three common insecticides(carbofuran, methyl parathion and parathion) used for the protection offruits were applied on the surface of an orange at 50 ppm concentration.Then CNT-coated paper was rubbed on the surface and held in front of theMS inlet for analysis. FIG. 9 panels A-C show molecular ion peaks ofdifferent pesticides using the battery powered spray MS method. It ispossible that the amount of sample extracted from the fruit surfacesduring rubbing may be several orders of magnitude lower than the appliedquantity, so the limit of detection may be much lower than the appliedsample concentration. FIG. 10 shows the molecular ion peaks of each ofthese pesticides from their mixture.

The same methodology was used to analyze medicines. CNT-coated paper wasrubbed on the surfaces of three commercially available medicinaltablets, viz. crocin, combiflam and xyzal (trade names) and held infront of a MS inlet with the 3 V battery set-up. FIG. 11 panels A-C showthat both crocin and combiflam contain paracetamol (acetaminophen) asthe major ingredient. Direct analysis of these tablets using CNT-coatedpaper gave a peak corresponding to protonated paracetamol. The othertablet, xyzal is a non-sedative antihistamine and contains levocetrizinedihydrochloride as the active ingredient. Analysis of this tablet (FIG.11 panel B) under the same conditions gave protonated levocetricine. Theidentity of the analytes was confirmed by MS² studies (data shown in theinsets).

Direct analysis of amino acids is also possible by spraying fromCNT-coated paper. Several amino acids (30 ppm) were dropped onto the tipof the CNT-coated paper with a micropipette (injected volume 3 μL,corresponding to a total loading of 10 ng). FIG. 12 panels A-H showintense protonated molecular peaks of the amino acids. The zwitterionicnature of amino acids may lead to easy extraction of ions from thenanotube tips in the electric field.

In order to probe the effect of the ionization event on the paperelectrode itself, Raman spectra of CNT-coated paper were recorded beforeand after a series of experiments (ionization of TPP at 3 V over aperiod of 20 minutes) (FIG. 13 panel A). See Costa et al., Mater.Sci.-Pol. 2008, 26, 433-441; and Naeemi et al., Annu. Rev. Mater. Res.2009, 39, 255-275. The data show a large red shift (Lee et al., J. Phys.Chem. Solids 2011, 72, 1101-1103; and Bhalerao et al., Phys. Rev. B:Condens. Matter Mater. Phys. 2012, 86) in the D and G bands whichimplies the acquisition of electrons in the CNT during positive modeionization (Scheibe et al., Mater. Charact. 2010, 61, 185-191). Itappears that as ionization occurs, charge builds up as would be expectedfor a field-assisted ionization (Luo et al., Chem. Phys. Lett. 2011,505, 126-129; and Goodsell et al., Arch., Phys. 2010, 1-12). However,there appears to be electron transfer from the developing microdropletto the CNT; effectively a polarization of the electrons in the long thinCNT fiber (see FIG. 1 panel D) driven by the high field and mobility ofelectrons in the CNTs and its large electron affinity (Shamsipur et al.,Electroanalysis 2012, 24, 357-367). As the positively charged dropletbreaks away, the residual charge appears to lead to reduction of CNTs asreflected in the Raman spectrum as red-shifted D and G bands. Thisspeculation on electron transfer from the charged microdroplet ofsolvent to CNT was supported by a blank experiment, where only solventand potential were applied to CNT-coated paper for the same period oftime and Raman measurement was performed. The spectrum showed red shiftsin D and G bands.

The Raman spectra of the nanotube sample were also recorded before andafter ionization of the salt tetramethylammonium bromide in both thepositive and negative ion modes. FIG. 13 panel B shows a red shift of Dand G bands for CNT-coated paper in case of the positive ion modemeasurement. This may be due as before to the high electric field neededto cause solvated ion ejection in microdroplets. There was no suchreduction for the negative ion mode measurement as Raman measurementsshow unshifted D and G bands, (FIG. 13 panel C) presumably because theCNTs are already electron-rich in this condition and the fieldreplenishes the charge lost.

The results presented here suggest a versatile strategy for the directanalysis of diverse chemical species. The methodology can be modified tosuit various analytical requirements. Replacement of the high voltagepower supply with a 3V battery simplifies mass spectrometry through ionformation from a nanoscale antenna. The CNT ionization method has beenapplied to a variety of samples from different sources including fruitsurfaces, medical tablets, and a range of organic molecules, includingamino acids, antibiotics and pesticides, at relatively lowconcentrations.

Example 3 Sample Analysis Using Probes of the Invention Using a 1V PowerSource

Low voltage (1 V) ionization using carbon nanotubes has been extended todetect various fragile species. These include a variety of acid dimersas well as hydrates of different anionic species. These fragile speciesare characterized by their low internal energy so that it can be easilydetected at low voltage, where fragmentation is negligible.

Five different acids from formic acid to pentanoic acid have beendetected in negative mode and the spectra are shown in FIG. 15 panelsA-E. Each spectrum is characterized by the presence of a proton bounddimer along with the molecular ion peak. Another finding was thedetection of mixed dimers at low voltage. Here various acid combinationswere made and were analyzed at low voltage of 1V. The results aredemonstrated in FIG. 16 panels A-D. Here, a variety of mixed dimers wereseen along with the expected individual acid dimers.

The variation of intensity of these acid dimers with voltage (D/M ratiovs voltage) has been tested and the results show a gradual decrease inthe intensity of acid dimers as we go to very high voltages. FIG. 17panels A-B represent the results, where D/M ratio has been plotted as afunction of voltage for five different acids in two different solvents(water and methanol). Similar study was carried out on a single acid indifferent solvents also. Here propionic acid was taken in differentsolvents and the D/M ratio was noted for a range of voltage from 1 V to3 kV. Result is shown in FIG. 18.

Another set of study was carried out at low voltage (1V) on differentanionic species where the aim was to detect various hydrates of anions.The results are shown in FIG. 19 panels A-D. The results show thepresence of various hydrates of different anionic species from chlorideto acetate.

Example 4 Spray Formation

This Example investigates the mechanism of low voltage spray. In orderto establish the mechanism of this spray two pathways were taken, e.g.,study of changing signal to noise ratio upon increasing low voltage tomoderately high voltage (500 V), secondly, by capturing video during thespray. Besides these work, some effort was given on computational studyto support the work. It is proposed that several mechanisms may beresponsible (operating independently or collectively) to give rise tosignal at very low voltage. Detection of microorganisms at low voltageis shown as an application of this method.

In order to understand the mechanism, a few experiments were performed.Analytes, such as, triphenylphosphine (PPh₃), tricyclohexylphosphine,dibutylamine, tributylamine were utilized in the experiments. One suchexample is given below.

In an experiment, 5 μL 50 ppm PPh₃ was sprayed into the massspectrometer. The spray voltages were varied from 2 V to 500 V. Thisinvestigation was stopped at 500 V for two reasons. Firstly, to avoidany discharge arising from the CNT coated paper tip. Secondly, above 500V spray voltage, the signal intensity increased rapidly and was reachingvalues comparable to normal paper spray.

The tip was held at a separation ˜0.5 mm from mass spec inlet. Spectrawere collected and analyzed and the signal to noise ratio (S/N) andsignal intensity with respect to increasing spray voltage was plotted(FIGS. 20 and 21). The distribution is divided in four regions. Region1, 2 V spray where the analysis was commenced, no signal. Uponincreasing the spray voltage to 3, 4 or 5 V, the S/N ratio shows rapidincrease (region 2). Region 3, 8-500 V, the S/N ratio is at steadystate. Beyond 500 V (region 4) the signal/noise rises rapidly but is notshown as it tends towards normal paper spray values. After 500 V, thesignal increases to two or three orders of magnitude compared to lowvoltage spray so normal paper spray (PS) comes into play at thisvoltage.

The appearance of analyte signal is controlled by two factors, fieldemission and field-induced transport of the ionized molecule (in a thinfilm of solution) to the high field (emitter) areas of the substrate. Ahypothesis is that at very low voltage (Region 1) the small number ofmolecules already in place at high field spots are ionized by fieldemission, presumably in microdroplets. This requires a limited voltagein combination with the small physical dimensions (order of 1 nm). So 5V at 1 nm gives a field strength of 5×10⁹ V/m which is just in the rangeof field emission (Beckey, Field Ionization Mass Spectrometry, Pergamon,London, 1971). Region 2 (5-300 V) is approximately a steady state regionin terms of S/N ratio in terms of signal there are two sub-regions,5-100 V where the signal falls and 100-300 V where it rises slightly.These data mean that either the field emission is slow, and ratelimiting, or the field induced transport is slow. One expects anincrease in field emission with voltage (if for no other reason than thefact that the area in which the field strength exceeds that needed forion formation is increasing). However, if transport is not effective,this signal can fall with increasing voltage because of prior removal ofmaterial from the higher field areas. It is proposed that the transportof material related to PPh₃ only becomes effective around 100 V. Thebreak at 300 V might be due to different mechanisms of transport, suchas solution phase vs. thin film or conversion to an ion here and veryeffective transport.

Example 5 Microorganism Analysis Using Probes of the Invention

For microorganisms, the bacteria isolates, supplied by bioMérieux, Inc.(Hazelwood, Mo.) were cultured from frozen samples stored at −80° C. onTSAB in cryotubes. All experiments were performed under InstitutionalReview Board guidelines IBC protocol #07-004-10 “Novel tissue,Biological fluid and Bacteria Evaluation by Mass Spectrometry” asamended. Five types of microorganisms were used in the present study,which are Escherichia coli (Gram-negative bacteria), Citrobacter farmeri(Gram-negative bacteria), Staphylococcus aureus (Gram-positivebacteria), Bacillus subtilis (Gram-positive bacteria), Saccharomycescerevisiae (yeast). These common microorganisms are chosen due to theirwidespread presence throughout the biosphere.

Microorganism samples (Bacillus subtilis in this case) were analyzedusing CNT spray and paper spray. Bacillus subtilis can be detected usinglow voltage spray using CNT coated paper and their mass spectra arecomparable (FIG. 22) to traditional paper spray. However, at 3 V, themass spectra intensity is usually three orders of magnitude less intensethan normal paper spray signal (FIG. 22 bottom panel). Reproducibilitywas tested by analyzing Citrobacter fameri seven times. In each massspectrum a similar relative abundances of lipids was obtained.

What is claimed is:
 1. A mass spectrometry probe comprising a substratein which a portion of the substrate is coated with a material, a portionof which protrudes from the substrate.
 2. The probe according to claim1, further comprising a voltage source coupled to the substrate.
 3. Theprobe according to claim 2, wherein the voltage source is configured togenerate a voltage of 3 volts or less.
 4. The probe according to claim1, wherein the material is an electrically conductive material.
 5. Theprobe according to claim 4, wherein the electrically conductive materialcomprises one or more electrically conductive nanotubes.
 6. The probeaccording to claim 5, wherein the electrically conductive nanotubes arecarbon nanotubes.
 7. The probe according to claim 6, wherein the carbonnanotubes coat an external surface of the substrate.
 8. A systemcomprising: a mass spectrometry probe comprising a substrate in which aportion of the substrate is coated with a material, a portion of whichprotrudes from the substrate; a voltage source coupled to the substrate;and a mass analyzer.
 9. The system according to claim 8, wherein thevoltage source is configured to generate a voltage of 3 volts or less.10. The system according to claim 8, wherein the material is anelectrically conductive material.
 11. The system according to claim 10,wherein the electrically conductive material comprises one or moreelectrically conductive nanotubes.
 12. The system according to claim 11,wherein the electrically conductive nanotubes are carbon nanotubes. 13.The system according to claim 12, wherein the carbon nanotubes coat anexternal surface of the substrate.
 14. The system according to claim 8,wherein the substrate tapers to a tip.
 15. The system according to claim8, wherein the probe is discrete from a flow of solvent.
 16. The systemaccording to claim 8, wherein the probe operates without pneumaticassistance.
 17. The system according to claim 8, wherein the massanalyzer is a miniature mass spectrometer.
 18. A system comprising: amass spectrometry probe comprising carbon nanotubes; a voltage sourcecoupled to the probe; and a mass analyzer.
 19. The system according toclaim 18, wherein the probe further comprises a substrate and the carbonnanotubes are coupled to the substrate.
 20. The system according toclaim 19, wherein the substrate is a porous substrate.